Operational tolerance to skin allografts in rats and mice has been studied as a model of tumor immunity. Previous studies from this laboratory have shown that animals bearing tolerated allografts, like animals with progressively growing neoplasms, demonstrate cell-mediated immunity and serum blocking activity specific for graft antigens. More recent studies using improved, quantitative MLC techiques have demonstrated significant reactivity in rats judged to be tolerant by the most stringent criteria. Moreover, lymphoid cells from tolerant rats have been shown to exhibit a synergistic interaction in mixtures with cells from normal rats. This evidence for persistent immunologic reactivity in highly tolerant rats must be contrasted with the results from other laboratories demonstrating a lack of such reactivity in highly tolerant mice. These apparently conflicting data may be reconciled by the assumption that operational tolerance may well be achieved by more than one immunologic mechanism, and that different mechanisms may be of relatively greater importance in one species or strain than another. The purpose of the proposed investigation is to better characterize the reactive lymphoid cells and serum blocking factors in operationally tolerant animals. This will be accomplished by the development of specific anti-rat T-cell reagents and by the identification of functionally distinct subpopulations of rat lymphoid cells using quantitative MLC and CML assays. Studies will be undertaken in H-2 recombinant mice to establish which antigens determine immunologic reactivity in tolerant hosts and to define which genetic mechanisms control responsiveness to these antigens. In vivo mechanisms of importance in transplantation tolerance will also be evaluated. These studies will critically test the hypothesis that subpopulations of lymphoid cells in tolerant animals may demonstrate different patterns of unresponsiveness to alloantigens.